A Draft Guideline for the Effective Use of Recycled Concrete Coarse Aggregate in Precast Concrete
Recycled Aggregates for Concrete: The Current Situation
Each year. Approximately 30 million tons of concrete structures are demolished. Innovative Materials and Resource Research Center (iMaRRC) is conducting research on how to reuse this demolished concrete as material for new concrete.
Concrete is made from gravel (coarse aggregate), sand (fine aggregate), water, and cement. The volume ratio of these materials is roughly 4:3:2:1(Photo 1). The idea of reusing demolished concrete as a substitute for gravel has existed for years. It is called recycled concrete aggregate. Japan Industrial Standards (JIS) are established for the quality of recycled concrete aggregate. There are three classes―H, M, and L―arranged in order from the highest quality (Photo 2). Class H consists almost entirely of natural aggregate extracted from demolished concrete and therefore has good quality. However, it takes a considerable amount of cost in order to remove original cement paste from the demolished concrete. In addition, gravel accounts for only about 40% of the material contained in the concrete. The other components become fine powder whose disposal can be problematic. Class L can be produced by crushing demolished concrete to an appropriate particle size. However, it cannot be used in structural concrete due to its inferior quality. Class M can be used in structures, but it is limited to areas that are less affected by weather (e.g., areas underground), and therefore its use is not widespread.
Outline of a Draft Guideline
In light of this situation, we prepared a draft guideline for the effective use of Calss M recycled aggregate in precast concrete (Figure 1). “Precast concrete” is a concrete product manufactured in a factory. This characteristic makes quality control easier and can diminish cracking risk by limiting product size even when using recycled aggregate concrete, which is prone to shrinkage due to drying. Additionally, we have confirmed in laboratory tests and outdoor exposure tests that precast concrete made with recycled aggregate possesses conditions that can ensure frost damage resistance equivalent to that of ordinary concrete even in cold regions where deicing salts are splayed(Figure 2, Photo 2)..
The draft guideline can be downloaded from https://www.pwri.go.jp/team/imarrc/research/tech-info.html. We hope it will provide useful in promoting the use of recycled concrete aggregate.
(Contact: Innovative Materials and Resources Research Center)